CyberSecBulletin
CVE-2025-39682Exploited in the wildNVD status: Analyzed

Linux Kernel Improper Check for Unusual or Exceptional Conditions

Published Updated By the CyberSecBulletin desk

CVE-2025-39682 is a critical-severity improper check for unusual or exceptional conditions vulnerability in Linux, with a CVSS 3.1 base score of 9.8 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 2902c3ebcca52ca845c03182000e8d71d3a5196f, c09dd3773b5950e9cfb6c9b9a5f6e36d06c62677, 3439c15ae91a517cf3c650ea15a8987699416ad9 as the first without the flaw, so updating Linux to one of those releases or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on September 18, 2026, which means attacks have been observed in the wild.

Patch now

Attackers are already exploiting this vulnerability, according to CISA.

What to do

  1. Update Linux to 2902c3ebcca52ca845c03182000e8d71d3a5196f or c09dd3773b5950e9cfb6c9b9a5f6e36d06c62677 or 3439c15ae91a517cf3c650ea15a8987699416ad9 or 29c0ce3c8cdb6dc5d61139c937f34cb888a6f42e, whichever matches your release line, or a later version.
  2. Update SIMATIC CN 4100 to V5.0 or a later version.
  3. Update Linux Kernel to 6.1.149 or 6.6.103 or 6.12.44 or 6.16.4, whichever matches your release line, or a later version.
  4. CISA's required action: Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
  5. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
LinuxLinux84c61fe1a75b4255df1e1e7c054c9e6d048da417 up to, but not including, 2902c3ebcca52ca845c03182000e8d71d3a5196f; 84c61fe1a75b4255df1e1e7c054c9e6d048da417 up to, but not including, c09dd3773b5950e9cfb6c9b9a5f6e36d06c62677; 84c61fe1a75b4255df1e1e7c054c9e6d048da417 up to, but not including, 3439c15ae91a517cf3c650ea15a8987699416ad9; 84c61fe1a75b4255df1e1e7c054c9e6d048da417 up to, but not including, 29c0ce3c8cdb6dc5d61139c937f34cb888a6f42e; 84c61fe1a75b4255df1e1e7c054c9e6d048da417 up to, but not including, 62708b9452f8eb77513115b17c4f8d1a22ebf843; 6.02902c3ebcca52ca845c03182000e8d71d3a5196f, c09dd3773b5950e9cfb6c9b9a5f6e36d06c62677, 3439c15ae91a517cf3c650ea15a8987699416ad9, 29c0ce3c8cdb6dc5d61139c937f34cb888a6f42e, 62708b9452f8eb77513115b17c4f8d1a22ebf843
SiemensSIMATIC CN 4100before V5.0V5.0
LinuxLinux Kernel6.0 up to, but not including, 6.1.149; 6.2 up to, but not including, 6.6.103; 6.7 up to, but not including, 6.12.44; 6.13 up to, but not including, 6.16.4; 6.176.1.149, 6.6.103, 6.12.44, 6.16.4
DebianDebian Linux11.0Not stated
SiemensSimatic Cn 4100 Firmwarebefore 5.05.0

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
HighAll data handled by the component can be read.
Integrity
HighData or code can be modified at will.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.

Scoring systemScoreSeverityScored by
CVSS 3.19.8CriticalCNA
CVSS 3.19.8CriticalNVD

Official description

In the Linux kernel, the following vulnerability has been resolved: tls: fix handling of zero-length records on the rx_list Each recvmsg() call must process either - only contiguous DATA records (any number of them) - one non-DATA record If the next record has different type than what has already been processed we break out of the main processing loop. If the record has already been decrypted (which may be the case for TLS 1.3 where we don't know type until decryption) we queue the pending record to the rx_list. Next recvmsg() will pick it up from there. Queuing the skb to rx_list after zero-copy decrypt is not possible, since in that case we decrypted directly to the user space buffer, and we don't have an skb to queue (darg.skb points to the ciphertext skb for access to metadata like length). Only data records are allowed zero-copy, and we break the processing loop after each non-data record. So we should never zero-copy and then find out that the record type has changed. The corner case we missed is when the initial record comes from rx_list, and it's zero length.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Improper Check for Unusual or Exceptional Conditions

The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.

How developers prevent this class of flaw

  • Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Choose languages with features such as exception handling that force the programmer to anticipate unusual conditions that may generate exceptions. Custom exceptions may need to be developed to handle unusual business-logic conditions. Be careful not to pass sensitive exceptions back to the user (CWE-209, CWE-248).
  • Check the results of all functions that return a value and verify that the value is expected.
  • If using exception handling, catch and throw specific exceptions instead of overly-general exceptions (CWE-396, CWE-397). Catch and handle exceptions as locally as possible so that exceptions do not propagate too far up the call stack (CWE-705). Avoid unchecked or uncaught exceptions where feasible (CWE-248).

More Improper Check for Unusual or Exceptional Conditions vulnerabilities (CWE-754)

Timeline

  1. CVE record published.
  2. Added to CISA's Known Exploited Vulnerabilities catalogue.
  3. CISA published its SSVC assessment (exploitation: active, automatable: yes, technical impact: total).
  4. Deadline for US federal agencies to remediate, set by CISA.
  5. Record last changed materially (score, affected products, status or references).

References

Recent Improper Check for Unusual or Exceptional Conditions vulnerabilities

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